The use of portable data processing systems has increased significantly in recent years. In addition to so called “laptop” and “tablet” computers, there is a growing popularity in handheld data processing devices, often called a “personal digital assistant” or “PDA.” All of these devices are capable of storing a significant amount of user data, including calendar, address book, tasks and numerous other types of data for business and personal use. Most handheld data processing devices have the ability to connect to a personal computer for data exchange, and many are equipped for wireless communications using, for example, conventional email messaging systems. Depending upon the user's needs much of this data can be highly sensitive in nature, especially for example in a government, military or commercial context.
Portable data processing systems are typically password protected. However, in the hands of a technically sophisticated individual with malicious intent, there are ways to obtain the data stored in memory in such a device if the data is not encrypted. Accordingly, the data stored in persistent memory is typically encrypted using an encryption key. The data is secure as long as the encryption key remains unknown.
There are two competing parameters in any high security embedded system. The first is to prevent an attacker from accessing secure data or taking control of the system. The second is to allow the system to be tested and debugged, both during development and after the product has been launched. For example, common debug mechanism available in embedded systems is known as the “JTAG” port, which can be used to communicate with the processor, and read from and write to memory locations, so that the processor and memory can be thoroughly tested.
However, there are cases when it is desired that the actual contents of memory not be known at any time, by software or by any other entity except for the hardware itself. Encryption key storage is an important example. To ensure system security, it is desirable that the encryption keys stored in memory never be known. At the same time, during manufacturing or at any other time when the memory needs to be tested, it is desirable to be able to control and monitor specific memory bit positions to verify their integrity and to isolate error locations.
One test approach is to have software perform writes, reads and compares to the memory location being tested, but this method reveals the memory contents to software, and thus potentially to an attacker who accesses the system through the debugging port. It would accordingly be advantageous to provide a method for testing a secure memory under software control, without revealing the memory contents.